The small-diameter drill in the title tends to have an increased core thickness in order to ensure the strength. For a drill having an increased core thickness, a method of reducing thrust force is adopted, by which cross thinning (also referred to as X thinning) is applied to the drill.
For a drill having an increased core thickness, cutting amount of a cutting edge is increased, and when the cutting edge of a flute (helical flute) portion is a straight line cutting edge, the radial rake angle (radial rake angle of the outer circumferential cutting edge) has a large negative angle, and thus an approach is adopted in which the flute portion cutting edge is curved in concave against a rotational direction of the drill so as to achieve a positive radial rake angle.
As a conventional example of a small-diameter drill having such a design, there is one disclosed in, for example, PTL 1 listed below,